Stray field electrode buffer



1959 v M. ROTHSTEIN ETAL 2,867,709

STRAY FIELD ELECTRODE BUFFER Filed June 14, 1956 2 Sheets-Sheet 1 FIG? FIG.

INVENTORfi Jan. 6, 1959 M. ROTHSTEIN ETAL 2,

STRAY FIELD ELECTRODE BUFFER Filed June 14, 1956 2 Sheets-Sheet 2 FIG. 3

STRAY FIELD ELECTRODE BUFFER Milton Rothstein and Bernard Lichtenstein, Flushing, N. Y., assignors to Radio Receptor Company, Inc., Brooklyn, N. Y., a corporation of New York Application June 14, 1956, Serial No. 591,319

Claims. or. 219-1053 This invention relates to the high frequency electric heating of dielectric materials and, more particularly, to an improved electrode buffer for use in the stray field heating of dielectric materials.

The most common arrangement for high frequency heating of dielectric materials is to place the material between a pair of electrodes connected to opposite terminals of a source of high frequency electric energy. However, in many cases, it is not possible to place the work between the electrodes, due either to the bulk of the work or other considerations. In such cases, the high frequency heating is effected by what is known to the art as stray field heating as contrasted to direct field heating.

In stray field heating, electrodes of opposite relative polarity are engaged with the same surface of the work and pressed thereagainst. A portion of the field between the electrodes thus passes through the work. However, as only such portion of the field is effective in heating the work, in contrast to substantially all the field being effective in direct field heating, higher voltages must be used to obtain an equivalent heating effect in the work.

Examples of heating by the stray field heating techinque' include the monument of panels to frames by high'fr'equency heating of the glue or other adhesive substance.

In many cases, the adhesive is squeezed out from between the work-pieces and sticks to the electrodes. This may cause arcing between the electrodes and damage thereto and to the work. In other instances, the requisite pressure on the electrodes is great enough to cause unsightly marks on the product. In still other cases, the pressure distribution is poor, as when the sectional modulus of a panel, between adjacent electrodes, is insufficient to transmit the electrode pressure to a frame to which the panel is to be secured.

In order to avoid these difiiculties, a buffer material is used between the electrodes and the work in stray field heating. This buffer prevents glue getting onto the electrodes, prevents the electrodes marking the work, and improves the electrode pressure distribution.

Nevertheless, the use of such buffers hitherto has introduced new problems. As the buffer absorbs power, there is a further decrease in the electrical efliciency and a change in the field distribution so that less of the field intersects the glue line. T o obtain the same heating rate with buffers as obtained without buffers, still higher voltages must be used. As the voltages used normally are of the same order of magnitude as the breakdown voltages across the material, there is an increased and considerable danger of arcing, with resultant damage to the electrodes and spoilage of the product.

In accordance with the present invention, these difiiculties are overcome by providing a buffer having an outer, or electrode-engaging, surface configured to provide increased air gap paths between adjacent electrodes of opposite polarity. These configurations not only considerably increase the length of the arc-over path between the electrodes but also, due to their high impedance to a United States Patent ice the high frequency field, deflect more of the field into the work for more effective heating of the latter.

In one embodiment of the invention, the surface configuration of the buffer comprises parallel, relatively deep grooves extending at an angle of 45 to the lengths of the electrodes. In another embodiment, the surface configuration comprises staggered holes or recesses preferably extending only part way through the buffer. For example, the holes may be along parallel lines running at 45 to the lengths of the electrodes.

For an understanding of the invention principles, reference is made to the following description of typical em bodiments thereof as illustrated in the accompanying drawings. In the drawings:

Fig. 1 is a partial plan view illustrating stray field dielectric heating electrodes arranged on a buffer incorporating the invention;

Fig. 2 is a sectional view on the line 22 of Fig. 1; v

Fig. 3 is a view, similar to Fig. 1, illustrating a modification of the invention buffer; and

Fig. 4 is a vertical sectional view illustrating a modified form of the buffer of Fig. 3, as applied to stray field heating the end of work-pieces.

Referring to Figs. 1 and 2, in stray field dielectric heating of work-pieces, a plurality of relatively elongated electrodes 10, 10 are arranged in spaced parallel relation. These electrodes are connected to opposite terminals of a source of high frequency electric energy such as an oscillator, for example. Alternate electrodes 10 are connected to one terminal of the source and intermediate terminals 10 to the other terminal of the source.

The work, which may, for example, comprise a panel 11 to be jointed to a frame 12 at a glue line 13, is heated from one side only by the field between electrodes 10, 10' passing through panel 11 and glue line 13 into frame 12. For this purpose, electrodes 10, 10 are pressed into firm contact with the outer surface of panel 11, with frame 12 being disposed on a suitable rigid support. As stated above, buffers are used between the electrodes and the work to prevent marring of the work by the electrodes, and the use of such buffers involves attendant disadvantages as to efliciency and arcing over of the electrodes.

Referring to Figs. 1 and 2, these disadvantages are avoided, in the present invention, by providing a buffer having an electrode-engaging surface in which the are over path between the electrodes is substantially increased. Thus, in the embodiment shown in these figures, buffer 20 has its electrode-engaging surface formed with a plurality of substantially parallel, relatively deep, and .preferably equispaced grooves 25 extending at an angle of preferably 45 to the longer edges of electrodes 10.

The shortest possible arc-over field path between electrodes 10 is indicated at A, this path being perpendicular to the longer edges of the electrodes. The arc-over field between adjacent electrodes would normally follow paths such as A if there were no grooves such as 25. However, as this path is interrupted by a groove 25, there is very little chance of arc-over therealong. The shortest practical path for arc-over is indicated at B, path B being uninterrupted by grooves 25. Path B can be made much longer than path A by proper choice of the width and spacing of grooves 25 and of the angle between the grooves and the electrodes. By so increasing the arc-over path, the possibility of arc-over is practically eliminated.

The provision of the air gaps of grooves 25 imposes a high impedance to the field flowing directly between the electrodes, and thereby a substantially increased part of the field is deflected into work 1011-12, thus increasing the electrical eificiency.

Fig. 3 shows an alternative arrangement in which the air gaps are formed by holes 35 in the electrode-engaging surface of buffer 30. While these holes can extend completely through electrode 30, the glue squeeze-out problem can be eliminated by extending holes 35 only part way through the electrode. Holes .35 are preferably arranged in rows at an angle, such as 45, to the electrodes.

The shortest path between electrodes is indicated at C, but this path is interrupted bythe air gaps at holes 35. The shortest path not interrupted by air gaps in the buffer is indicated at D, and this path is substantially longer than path C.

In both cases, buffers 20, 30 has the same mechanical advantages as the usual buffer in making the pressure application more uniform, in preventing marking of the work, and in preventing squeezed-out glue getting on the electrodes.

Fig. 4 illustrates another advantage of using a buffer. In a practical dielectric heating apparatus, the electrode spacing is fixed. Since various sizes of product are used, the fixed electrode spacing cannot always be set so as to exactly match-up with the product. Consequently, at the ends of the product in such cases, very little pressure can be exerted. However,thcuse of a buffer such as 29 or 30 overcomes this difiiculty and provides uniform pressures even at the ends of the product.

In Fig. 4, buffer 30' is generally similar to buffer 30 of Fig. 3, but holes 35' extend completely through the buffer and a sheet of paper or cardboard 40 is ,placed between the bu'fferand the work to prevent glue squeezing out onto the electrodes.

While specific embodiments of the invention have been shown and described in detail to'illustrate 'the application of the invention principles, it will be understood that the invention may be embodied otherwise'without departing from such principles.

What is claimed is:

1. For use in high frequency electric field heating of work pieces 'by the stray fields between substantially parallel elongated electrodes engaged with .the same surface of the work; a'buffer'of dielectric material interposed between the 'electrode'sand the work; the surface of the buffer directly engaged by the electrodes being interrupted by spaced depressions to provide air gaps therein between adjacent electrodes of opposite polarity, and the air gaps 'ibeing in the shortest physical distance between adjacent electrodes to increase the length of the arc-over path between the electrodes.

2. A buffer as claimed in claim 1 in which the air gaps are formed by regularly spaced relatively deep depressions in such surface of the buffer.

3. A buffer as claimed vin claim 1 in which the air gaps are formed by relatively deep, substantially parallel groves in such surface of the electrode.

4. A buffer as claimed in claim 1 in which the air gaps are formed by relatively deep, substantially parallel grooves .in such electrode engaging surface, the grooves extending at an angle to the lengths of the electrodes.

5. Abuffer as claimed in claim 1 in which the air gaps are formed by relatively deep, substantially parallel grooves in such electrode engaging surface, the grooves extending at an angle of substantially to the lengths of the electrodes.

6. A buffer as claimed in claim '1 in which the air gaps are formed by holes in such surface of the buffer.

7. A buffer as claimed in claim 1 in which the air gaps are formed by holes in such surface ofthebuffer extending through the buffer.

trodes.

10. A buffer as claimed in claim 1 in which the air gaps are formed by holes'in such surface of the buffer extending part way through the buffer, the holes being arranged in rows extending at an angle of substantially 45 to the lengths of the-electrodes.

References Cited in the file of this patent UNITED STATES PATENTS Hart Apr. '15, 1952 Mann Jan. 20, 1953 

